This invention relates to a method for purifying industrially critical metals, such as Fe, Co, Ni or Cu to metal of high purity by removing minor amounts of impurities contained in those metals, such as light elements, alkali metals, alkali earth metals or ceramics. The present invention also relates to a method for refining the metals.
Metals such as Fe, Co, Ni or Cu are used as materials of electronics or as functional materials extensively. In particular, Fe and,Co are used for a variety of recording mediums exploiting characteristics as a ferromagnetic material, a permanent magnet material or as a positive electrode material for lithium ion cells.
An evaporated tape for recording digital signals, a photomagnetic optical recording medium exemplified by MO or MD and a magnetic recording medium exemplified by a hard disc are enumerated as recording medium. As the permanent magnet material, SmCo or NdFeB is noteworthy. As a matter of course, these materials are desirable of high purity.
From now on, it is quite within the bounds of possibility that metals such as Fe, Co, Ni or Cu will be used as starting material for LSI. In these years, investigation of sputtering of these metals are going on briskly. For example, if Co is used as an electrode material, the demand for a CoSix (where x greater than O) target is increased, such that high purity Co becomes more and more crucial for industrial application. Also, Fe, which is expected as a material of semiconductor working in infrared range will become more critical in time to come.
Thus, metals used for semiconductors, such as Fe, Co, Ni or Cu are required to be higher in purity than the materials used for the above recording mediums. For example, there is raised a strong demand for reducing the content of alkali metal impurities deteriorating characteristics of MOS devices, such as Na, alkaline earth metal impurities, such as Mg or Ca, or of radioactive impurity elements radiating xcex1-rays to give rise to malfunctions, such as U or Th.
Under the above-described technical background, the demand for high-purity Fe, Co, Ni or Cu is considered to be increasing in time to come.
So, prompt and efficient removal of oxygen is imperative for regenerating and improving the purity of these metals. This removal of the large quantity of oxygen is equivalent in principle to routine metal refining of removing oxygen from metal oxides to yield metal. However, there is not known up to now a technique of effectively recycling these used Fe, Co, Ni or Cu from the viewpoint of economy and maintenance of earth environment.
Thus, in these years, a technique of refining raw metals or used Fe, Co, Ni or Cu metal to high purity, in particular a technique which is easy to work out, economically meritorious and more amicable to environment, has been desired.
Heretofore, the prevalent way to reduce metal oxides takes much time, such as
(i) a method of reducing the metal oxides by dry refining, using reducing agents, such as C, Al or Mg;
(ii) a method of transiently dissolving metal oxides in an aqueous solution for recovering metals on electrolysis; or
(iii) a method of elevating the temperature in a reducing atmosphere, such as in a hydrogen stream, for reducing the metal oxides.
In the method (i) above, Al or Ca, exhibiting higher affinity to oxygen than the metals, is molten as reducing agents along with metal and removed from the metals as Al2O3 or CaO.
If, in this case, the amount of oxygen contained in the metal is not estimated elaborately in advance, there are raised such problems that the oxygen cannot be removed sufficiently depending on the charged amount of the reducing agent or that conversely the excess reducing agent, such as Al or Ca, be left over as impurities.
In the method (ii) above, H2 or Cl2 gases are inevitably generated as reaction products if a HCI Bath is used as a solvent. The method shown in (iii) above has a drawback that the reaction temperature is extremely high such that a high energy is required.
In addition to the aforementioned oxygen removing method, the following process is known as a method for removing metal and non-metal impurities.
The alternative method is to melt an impure metal with electron beam in vacuum of 10xe2x88x922 to 10xe2x88x924 Pa, where the impurities will be evaporated and removed because of the difference of vapor pressure between the impurities and matrix metals. The problem encountered in this method is that an evacuating device of large displacement is needed and that the vacuum needs to be maintained for prolonged time thus increasing the size of the device. Moreover, if the amount of metal impurities is to be reduced to a smallest amount possible, prolonged melting is mandatory, with the result that the loss of metals due to evaporation is increased, thus lowering the yield.
Moreover, with the above-described technique, radioactive element impurities, such as U or Th, are difficult to remove. If desired to remove these radioactive element impurities, such as U or Th, the metals need to be dissolved in aqueous solutions and wet processes, such as ion exchange methods or solvent extraction methods, need to be executed. These wet processes are uneconomical since the required space is tens to hundreds times of the unit processing volume as compared to the dry process exemplified by the melting method. The method of recovering metals from the metal-containing solution, purified by the wet process, such as the electrolysis or the method of evaporating the solution to dryness to recover metal salts and processing the metal salts to solid-phase hydrogen reduction, is also extremely time and energy consuming, and hence uneconomical.
It is therefore an object of the present invention to provide a complete recycling process in which high-purity metals, such as Fe, Co, Ni or Cu, can be easily purified and recovered without excessively increasing the size of the purification and refining device or its ancillary devices or excessively complicating the operation.
The present inventors have conducted prolonged and perseverant investigations towards resolving the above-mentioned technical problems, and found that, by melting metals, such as Fe, Co, Ni or Cu with the hydrogen plasma arc melting method or the hydrogen atmosphere arc melting method, trace amounts of alkali metal impurities, such as Na, alkaline earth metal impurities, such as Ca, non-metal impurities, such as oxygen, nitrogen or carbon, or radioactive element impurities, such as U or Th, contained in metals, such as Fe, Co, Ni or Cu, can be promptly removed on vaporization by the sole melting process. This information has led to completion of the present invention.
The present inventors have also elaborated the information that oxygen in metal can be removed by the hydrogen plasma arc melting method or the hydrogen atmosphere arc melting method, and found that these methods can be applied to the method of refining high-purity oxides of Fe, Co, Ni or Cu to high-purity Fe, Co, Ni or Cu metals. This finding also has led to completion of the present invention.
In one aspect, the present invention provides a method for purifying metals, wherein metals containing impurities are molten by an argon-hydrogen plasma arc containing active hydrogen H to remove the impurities.
In another aspect, the present invention provides a method for refining metals wherein metals including ceramics inclusions are molten by an argon-hydrogen plasma arc containing active hydrogen H, wherein the ceramics inclusions are floated over molten metal because of difference of density between the molten metal and the ceramics inclusions, and wherein the floating ceramics inclusions are decomposed and removed.
In yet another aspect, the present invention provides a method for refining metals wherein metal oxides are molten by an argon-hydrogen plasma arc containing active hydrogen H for reducing the metal oxides to metals.
The basic concept underlying the above-mentioned respective aspects of the present invention is the use of the hydrogen plasma arc melting method or the hydrogen atmosphere arc melting method for refining metals, such as Fe, Co, Ni or Cu. This effectively removes non-metallic impurities, such as oxygen, nitrogen or carbon to enable refining and recovery of high-purity metals containing these impurities at an extremely low level.
The refining process defined above is clean and amenable to earth environment since the time of reducing reaction per unit volume is extremely short and no reaction products other than the metals and H2O (+Ar) are yielded.
Similarly, with the refining method according to the present invention, there may be provided a process for reducing metal oxides which is clean and devoid of by-produced CO or CO2 in comparison with the conventional technique. The refining process according to the present invention is not only economical but also amenable to environment.
Thus, according to the present invention, there may be provided a purification and refining methods whereby high-purity metals (Fe, Co, Ni and Cu) can easily be purified and recovered, with the possibility of complete recycling inclusive of regeneration and re-utilization thereof, without excessively increasing the size of the purification and refining device or its ancillary devices or excessively complicating the operation.
In a first embodiment, the present invention provides a method for purifying metals that comprises melting at least one metal containing at least one impurity in an argon-hydrogen plasma arc further comprising active hydrogen H to remove said impurities.
In the first embodiment, said metal comprises at least one metal selected from the group consisting of Fe, Co, Ni and Cu.
In the first embodiment, said plasma arc contains argon as a generating gas.
In the first embodiment, said impurities comprise at least one selected from the group consisting of alkali metal impurities, alkaline earth metal impurities, non-metallic impurities and radioactive element impurities.
In the first embodiment, the non-metallic impurities comprise at least one impurity selected from the group consisting of oxygen, nitrogen and carbon.
In the first embodiment, the hydrogen in the plasma arc generating gas is present in an amount ranging from about 0.05 vol % to about 100 vol %.
In the first embodiment, the melting step is conducted in a furnace having a pressure ranging from about 1.33 kPa to about 310 kPa.
In a second embodiment, the present invention provides a method for refining metals that comprises melting at least one metal comprising at least one ceramic in an argon-hydrogen plasma arc further comprising active hydrogen H; floating said ceramic over molten metal by utilizing differences in density between the molten metal and the ceramic; and decomposing and removing the floating ceramic.
In the second embodiment, said metal comprises at least one metal selected from the group consisting of Fe, Co, Ni and Cu.
In the second embodiment, the hydrogen is present in the plasma arc generating gas in an amount ranging from about 0.05 vol % to about 100 vol %.
In the second embodiment, the melting, floating and decomposing and removing steps are carried out in a furnace having a pressure ranging from about 1.33 kPa to about 310 kPa.
In a third embodiment, the present invention provides a method for refining metals that comprises melting at least one metal oxide in an argon-hydrogen plasma arc further comprising active hydrogen H for reducing the at least one metal oxide to a metal.
In the third embodiment, said metal comprises at least one metal selected from the group consisting of Fe, Co, Ni and Cu.
In the third embodiment, the hydrogen is present in the plasma arc generating gas in an amount ranging from about 0.05 vol % to about 100 vol %.
In the third embodiment, the melting step is carried out in a furnace having a pressure ranging from about 1.33 kPa to about 310 kPa.
Other objects and advantages of the present invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.